17:15 〜 18:30
[SCG39-P02] 琉球海溝南西部における超低周波地震とプレート間カップリング領域の相補的分布
キーワード：スロー地震、琉球海溝、プレート間カップリング、超低周波地震
We investigated the distribution of very low-frequency earthquakes (VLFEs) in the southwestern Ryukyu Trench and clarified its relationship with the interplate-coupling area. In the southwest Ryukyu Trench, VLFEs are widely distributed from the Yaeyama Islands to near Taiwan (Nakamura and Sunagawa, 2015). In this region, slow slip events (SSEs), interplate locked zones (Hsu et al., 2012), and source fault of tsunami earthquake (Nakamura, 2009) are distributed. Distribution of these and VLFEs is important for understanding the frictional conditions in the plate interface subducted in the southwest Ryukyu Trench. However, the broadband seismic network (Fnet) of the National Research Institute for Earth Science and Disaster Prevention (NIED), which has been used in previous studies, has a linearly biased arrangement of seismic stations, which tends to cause large deviations in the determination of the epicenter. Furthermore, the error in determining the epicenter was as large as several tens of kilometers. In order to improve the accuracy of the location, the epicenters of the VLFEs were determined by adding data from Taiwan's Broadband Active Seismic Network (BATS) and temporary broadband seismometers by Association for the Development of Earthquake Prediction to Fnet data and using the semblance method and template method.
We used the semblance method to determine the locations of VLFEs using data for the periods 2005-2013 and 2015-2018. The template method used the data from the period January 2005 to December 2018 to determine the position of the VLFEs. A band-pass filter of 0.02-0.05 Hz was applied to the vertical component waveform in both cases. In the template method, a thrust-type earthquake south of Iriomote Island was used as a template.
The epicenters of the VLFEs determined by the semblance method were distributed in an elongated region along the trench axis. The VLFEs were further divided into three clusters. The clusters were distributed off the south coast of Miyako Island, off the southeast coast of Ishigaki Island, and the south coast of Iriomote Island and Yonaguni Island. The cluster off the southern coast of Iriomote Island and Yonaguni Island had a shape that extends along the trench axis. The slab depths at these epicenters were 10-15 km. The distribution of VLFEs determined by the template method was similar to the case of semblance method, with the epicenter of VLFEs distributed in an elongated region along the trench axis. The epicenter's distribution determined by the template method was 10-20 km north of the one by the semblance method. However, we could detect the clusters off the southern coast of Iriomote Island and Yonaguni Island in the template method. The distribution of clusters off the southeastern coast of Ishigaki Island was scattered. We determined the CMT solutions of the VLFEs and found that they were all reverse fault earthquakes that occurred at epicentral depths of 17-35 km.
The distribution and depths of the determined VLFEs were similar to those of the low-frequency earthquakes (LFEs) determined by OBS (Arai et al., 2016). It was also compatible with the location of LFEs accompanying with VLFEs (Nakamura, 2017). The VLFEs did not occur in the locked area in the southwestern part of the Ryukyu Trench. Similarly, it did not overlap with the region of the reverse fault earthquakes on the down-dip side of the locked area. This suggests that the areas where VLFEs and LFEs occur have a complementary relationship with the interplate locked area.
We used the semblance method to determine the locations of VLFEs using data for the periods 2005-2013 and 2015-2018. The template method used the data from the period January 2005 to December 2018 to determine the position of the VLFEs. A band-pass filter of 0.02-0.05 Hz was applied to the vertical component waveform in both cases. In the template method, a thrust-type earthquake south of Iriomote Island was used as a template.
The epicenters of the VLFEs determined by the semblance method were distributed in an elongated region along the trench axis. The VLFEs were further divided into three clusters. The clusters were distributed off the south coast of Miyako Island, off the southeast coast of Ishigaki Island, and the south coast of Iriomote Island and Yonaguni Island. The cluster off the southern coast of Iriomote Island and Yonaguni Island had a shape that extends along the trench axis. The slab depths at these epicenters were 10-15 km. The distribution of VLFEs determined by the template method was similar to the case of semblance method, with the epicenter of VLFEs distributed in an elongated region along the trench axis. The epicenter's distribution determined by the template method was 10-20 km north of the one by the semblance method. However, we could detect the clusters off the southern coast of Iriomote Island and Yonaguni Island in the template method. The distribution of clusters off the southeastern coast of Ishigaki Island was scattered. We determined the CMT solutions of the VLFEs and found that they were all reverse fault earthquakes that occurred at epicentral depths of 17-35 km.
The distribution and depths of the determined VLFEs were similar to those of the low-frequency earthquakes (LFEs) determined by OBS (Arai et al., 2016). It was also compatible with the location of LFEs accompanying with VLFEs (Nakamura, 2017). The VLFEs did not occur in the locked area in the southwestern part of the Ryukyu Trench. Similarly, it did not overlap with the region of the reverse fault earthquakes on the down-dip side of the locked area. This suggests that the areas where VLFEs and LFEs occur have a complementary relationship with the interplate locked area.